docs: reorganize into project/ + reference/, expand Storybook Foundations

Move working docs (backlog, prd, roadmap) under docs/project/ and durable
docs (architecture ADRs, guides, audits) under docs/reference/; add a
docs/README.md index. Update every path reference in code comments, CLAUDE.md,
READMEs, and the new-ssp skill. Expand the Storybook Foundations curriculum
(Overview, BDD, i18n; rename Layers→Domain-Driven Design) and reorder the sidebar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/project/backlog/WP-24-letter-canvas.md

@@ -15,7 +15,7 @@ domain model, `brief.machine.ts`, and every `BriefMsg` stay byte-identical.
 - PRD Brief v2 §2b (fidelity note), §4, §10; the sample `voorbeeldbrief-inschrijving.pdf`
 - `src/app/brief/ui/letter-composer/letter-composer.component.ts` (the `canEdit` pivot)
 - `src/app/brief/ui/letter-preview/letter-preview.component.ts` (rendering that migrates in)
-- `docs/backlog/WP-23-org-template-backend.md` (the `orgTemplate` on `BriefViewDto`)
+- `docs/project/backlog/WP-23-org-template-backend.md` (the `orgTemplate` on `BriefViewDto`)
 
 ## Decisions (pre-made, don't relitigate)